1. Field of the Invention
The present invention relates to a light communication system. In particular, the present invention is directed to a device that enables a transmission between an optical fiber and a light receptor, and, specifically, an optical transmission device that is used to transmit a signal between an optical fiber and a light receptor.
2. Description of Background Information
Generally, two main types of optical fiber cables are employed; glass optical fibers and plastic optical fibers.
Glass optical fibers have a higher transparency, as compared to plastic optical fibers. When the precision of the light transmission is to be improved, the diameter of the core part of the optical fiber is designed to be as small as possible. As a result, the ability of the glass optical fiber to carry a single mode signal is enhanced.
On the other hand, plastic optical fibers offer a better resistance to bending and countermeasures to an optical interference or attenuation in the fiber can be envisaged. The diameter of a plastic optical fiber is usually greater than the diameter of a glass optical fiber. Due to the larger diameter, plastic optical fibers tend to suffer from a certain level of mode dispersion, thereby causing light transmission loss and wave deformation. Nonetheless, due to the relatively large diameter of the optical fiber, high precision alignment of one optical fiber to another optical fiber (or an optical fiber to an element, such as, for example, a receptor) is not required. Plastic optical fibers are therefore easily optically aligned, are less costly than glass optical fibers, and thus, are used in optical communications devices. Because of its large diameter, a known plastic fiber can receive a certain number of optical signals emitted by a light emitting diode (LED) into this optical fiber. These optical signals may then exit and be received by a large chip-size photodiode (PD), which transforms the optical signals into electrical signals.
In recent optical fiber-communications, research has taken place into high speed optical communications having a speed of, for example, several hundreds of Mbps (megabits per second) to an order of several Gbps (gigabits per second). A surface of a receptor-side photodiode, to be irradiated by a light to generate excited electrons, has a limited chip size. The electrons move to a position located at a chip terminal, where wires are bonded. The greater the chip surface of the photodiode, the longer it takes for the electrons to move from one point to another point (e.g., electron movement time increases). Therefore, the chip surface of the photodiode should be designed to be as small as possible.
However, in the case of a photodiode having a small surface chip, the known plastic optical fibers have been designed to have a large opening diameter, as mentioned above. As a result, as shown in FIG. 1 of the drawings, only part of the signal coming out of light-exit end face A2 of the fiber optic (optical fiber) A1 is received by the photodiode A3. Thus, the optical loss is increased between the light-exit end face A2 and the photodiode A3.
When the plastic optical fiber has, for example, an end external diameter of 750 .mu.m, a uniform light can be emitted therefrom and received by a photodiode having a light-reception diameter face of 250 .mu.m. If calculated on the basis of the surface ratio, only 11% of the original light volume is received. This means that, when transformed into decibel milliwatt (dBm) ratings, the light volume is decreased by 9.54 dBm.
Thus, a large open diameter plastic fiber A1, though easily handled, is poorly adapted to a photodiode A.sub.3 having a small chip surface designed for receiving a high speed signal, as a large loss occurs due to the bonding of a large diameter optical fiber to a small diameter photodiode.
Accordingly, a purpose of the present invention is to provide an optical-transmission device which, when used with an optical receptor having a small surface chip, reduces the optical loss due to bonding.